1. Field of the Invention
The invention relates to a magnetic resonance imaging apparatus, comprising a magnet system for generating a steady magnetic field in a measuring space, a gradient coil system for generating gradient fields in the measuring space, a power supply source for the gradient coils, and a communication system for transferring acoustic information from at least a first region in which the level of sounds generated by the gradient coils referred to herein as "gradient noise" is comparatively high to at least a second region, which communication system comprises means for generating a reference signal which is dependent on the gradient noise, a microphone which is arranged in the first region so as to pick up a mixture of sound information and gradient noise, a sound reproduction device, at least a part of which is situated in the second region, and a noise suppression device which comprises a filter device for converting the reference signal into a signal which corresponds substantially to the gradient noise at the area of the microphone, and a summing device for adding the output signal of the filter device to the output signal of the microphone in phase opposition, the output of the summing device being connected to the sound reproduction device.
2. Description of the Related Art
U.S. Pat. No. 5,033,082 discloses a communication system with active noise cancellation which is suitable for various applications, one of the feasible applications mentioned being an application in a magnetic resonance imaging apparatus. As is known, during operation the gradient coils in such an apparatus produce an annoying noise which strongly impedes the communication between a patient being examined in the apparatus and personnel around the apparatus. The known communication system is capable of improving this situation, but it has been found that the result still is not optimum. For example, when the gradient coils are activated in a non-periodic manner (for example, in the case of quickly changing preparatory gradients, a non-linear profile sequence, changing slice orientations), the noise cancellation device cannot follow the noise signals caused by the gradient coils, so that the noise cancellation is either lacking or very incomplete. Moreover, in the second region disturbing noise may occur which is not compensated by the known device and which, in conformity with the cited document, requires a separate noise cancellation device which renders the overall device substantially more complex and expensive.
It is an object of the invention to provide a magnetic resonance imaging apparatus of the kind set forth in which the communication system is simpler and more effective than the known system. To this end, the device in accordance with the invention is characterized in that between the microphone and the summing device there are provided signal delay means for delaying the microphone signal for a predetermined period of time, and that the sound reproduction device comprises means for attenuating sound which does not originate from the sound reproduction device.
The invention is based on the idea that substantially complete suppression of (usually non-periodic) gradient noise is possible only if the reference signal is added to the output signal of the microphone exactly at the correct instant (with the correct phase and amplitude). The reference signal in the known device will generally be slightly too late so as to enable full compensation. Because the microphone signal is also delayed in accordance with the invention, the reference signal can arrive exactly on time again. Thus, the sound reproduction device reproduces the sound information substantially without noise. Should disturbing noise also occur in the second region, caused by the gradient coils or by other sources of noise, therefore, it suffices to ensure that this noise cannot reach the ear of the listener. This is very simply realised by providing means in accordance with the invention which attenuate sound which does not originate from the sound reproduction device.
A preferred embodiment of the apparatus in accordance with the invention is characterized in that the sound reproduction device comprises a headset with a pair of earphones which are embedded in a sound-absorbing material. This embodiment offers the advantage that the attenuation of the ambient sound is achieved by means of very simple steps and that the person wearing the headset has a given freedom of movement. This is the case notably when the headset is of the wireless type.
Most types of headset are connected to an amplifier via electrically conductive wires. Because it generally is undesirable for electrical conductors to extend into the measuring space from the outside, an embodiment of the apparatus in which the second region is at least partly coincident with the measuring space is characterized in that the sound reproduction device comprises an electro-acoustic transducer which is arranged outside the measuring space and which is acoustically connected, via at least an air-filled tubular connecting member, to sound reproduction members which are enclosed by a sound-absorbing material and form part of a head section which can be arranged on the head of a patient in the measuring space. In this embodiment the advantages of the use of a headset are obtained without incurring the drawbacks of electric conductors extending into the measuring space.
A further embodiment is characterized in that the means for generating the reference signal are arranged to receive on their input a signal which corresponds to the output signal of the power supply source for the gradient coils. This embodiment utilizes the idea that the signals presented to the gradient coils are directly related to the gradient noise produced by these coils. Thus, these signals contain advance knowledge concerning the gradient noise so that they are particularly suitable to act as the basis for forming the reference signal. Should for some reason this advance knowledge not be used, the reference signal can also be obtained in a different manner, for example in that the means for generating the reference signal comprise a second microphone which is arranged so that it can pick up the gradient noise.